1. Field of Invention
The present invention relates to a method to illuminate a curtain of liquid droplets so that natural primary and secondary rainbows can be observed continuously from opposite sides of the droplet curtain.
2. Description of the Prior Art
The prior art has not attempted to facilitate the production of actual rainbows through a curtain of liquid droplets as might be provided by means of a water fountain. Rather, the prior art has attempted to simulate rainbows by entirely different optical processes, such as by using colors produced by absorption rather than refractive dispersion. The reasons for both the difficulties experienced by prior art and its resulting compromises are more easily understood after an explanation as to the manner in which rainbows are produced in nature. Against this background, the solution to the problem of artificial rainbow production provided by the present invention stands in stark contrast to the efforts of prior art.
Rainbows are produced in nature when nearly parallel beams of light from either the sun or the moon are scattered by the nearly spherical droplets of water in a rain shower, spray from a waterfall, or an artificial source of water droplets such as provided by a water fountain. The position of each rainbow is at a fixed angular distance from the light source: approximately 138 degrees for the primary rainbow and approximately 129 degrees for the secondary rainbow. As a consequence, the resulting rainbow can only be seen from one side of the shower or fountain. If, for example, an observer to the west of the shower can see a rainbow in the shower, an observer to the east of that shower cannot see the rainbow. Another consequence is that as the sun, or moon, moves across the sky, the position of each rainbow moves with it. Further, rainbows may not be visible if the sun or moon are high in the sky, such as is the case in midday in the mid-latitudes.
Because the object of the present invention is to enable a rainbow to be viewed continuously in a curtain of water droplets, the explanation of rainbow production herein will be confined to the behavior of rainbows in such curtains. For convenience, references to the sun will be taken to mean either the sun or the moon.
The observation of natural rainbows in a curtain of water droplet depends on the natural occurrence of many factors. Often, a rainbow cannot be observed as a result of cloudy or foggy conditions. In such conditions, the requisite parallel light from the sun has been replaced with ineffective diffuse light from the clouds. Even when the sun is out, it is often not in the proper position to enable an observer to see either a primary or secondary rainbow. On the few occasions when a rainbow can be observed, its position in the fountain and the portion of the rainbow which can be observed is variable even for a fixed observer.
The most obvious solution to the problem of rainbow production would appear to be to provide a substitute sun--a fixed source of essentially parallel light which would illuminate the fountain. However, such a solution would require, at one extreme, a bank of collimated lights with an area roughly as large as that of one face of the curtain of droplets itself, or, at the other extreme, a single powerful collimated light placed at a great distance from the curtain and having a comparable brightness and angular size (as observed from the curtain) as the sun. Neither solution lends itself practically or aesthetically to the construction of a rainbow fountain for viewing by the public. In addition to the ungainly size of the facilities required by both solutions, the fact that an observer would be required to stare directly into these brilliant lights if he were to turn his back to the fountain eliminates such solutions for practical use with the public.
Several prior art efforts have been attempted to address the problem for artificial rainbow production. U.S. Pat. No. 4,681,402 to Carlton R. Tiffany discloses a rainbow projector. The Tiffany patent discloses a method by which an arc of colors resembling a natural rainbow can be projected upon a screen by using a curved prism. As Tiffany's screen might conceivably be a curtain of water droplets such as obtained from a water fountain, the Tiffany device might provide a rainbow-like appearance upon the fountain. Similarly, U.S. Pat. No. 4,557,055 to Akira Arai discloses a rainbow projector, based upon a series of prisms and lenses, which conceivably might be used to project an arc of rainbow-like colors upon a curtain of water droplets, although Arai's intent was to use a more conventional surface such as a room wall. However, neither inventions, even if used with a fountain, would produce real rainbows as the colors are produced by an external device and merely projected to a display medium, such as a screen, rather than having the colors being produced by refractive dispersion within the water droplets themselves.
U.S. Pat. No. 4,002,333 to Hideyuki Gotoh discloses a rainbow phenomenon developing device. The Gotoh patent discloses a device by which a wall of falling water droplets is illuminated by colored panels which are arranged to provide a rainbow-like appearance. The result, however, is not a rainbow, as the colors were produced by absorption in the panels rather than being produced by refractive dispersion within the water droplets themselves.